Nozzle and bucket stages for steam turbines have for some time been the subject of substantial developmental work. This is because the efficiency of the power plant cycle is largely dependent on the efficiency of the energy conversion in the turbine. Thus, in is highly desirable to optimize the performance of steam turbine nozzles and buckets to improve aerodynamic efficiency, particularly by minimizing aerodynamic and steam leakage losses. In a typical nozzle design, there is a substantially linear distribution of the flow velocity leaving the nozzle exit. The nozzle leaving angle is the angle between the flow angle and a plane normal to the machine or turbine axis. This angle typically changes in a linear manner from the root to the tip, for example, on the order of 12.degree. to 15.degree.. In a typical bucket design, the total velocity at the bucket exit is substantially constant, i.e., there is no flow shifting from root to tip, or vice-versa. Additionally, the bucket leaving angle .DELTA. i.e., the angle at which flow exits the bucket relative co the axis of the machine or turbine, is substantially fairly constant from tip to root for a typical stage having a free vortex design.
Present nozzle designs typically include a large number of nozzles to avoid excitation of bucket resonant modes. Because of the high nozzle count, nozzle blades having extended noses for structural strength purposes are often provided. This is turn results in efficiency-lowering high surface friction forces. A lower solidity nozzle is thus desirable to increase turbine stage performance.
While these characteristics of nozzle and bucket designs as described are quite efficient aerodynamically, the present invention provides still further improved aerodynamic efficiencies, improving the overall performance of the turbine.